1. Field of the Invention
The present invention relates to a developing unit to be used in an electrophotographic image forming apparatus and to an electrophotographic image forming apparatus.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a developing unit serves to supply toner on a developing roller to an electrostatic latent image on an electrophotographic photosensitive member to form a toner image. The developing roller is rotationally driven while a toner regulating member is brought into abutment therewith, and a toner layer whose toner charge quantity has been controlled is formed on the developing roller.
In association with the development of a computer and multimedia, process for outputting a high-definition full-color image has been demanded in a wide variety of fields ranging from offices to households in recent years. Accordingly, additional improvements in speed, image quality, and durability have been required. In the developing unit, the developing roller and the toner have been improved for suppressing a reduction in developability due to the deformation of the toner caused by a stress which the toner receives in the developing unit.
For example, the following technology has been disclosed (Japanese Patent Application Laid-Open No. 2005-141192). Flexibility is imparted to the developing roller by forming a surface layer having a polyurethane obtained by polymerizing a polyurethane polyol prepolymer and an isocyanate compound, and hence the stress to be applied to the toner is reduced.
Meanwhile, the following toner particles each having a core-shell structure are preferably used in the toner for preventing the reduction of the developability due to the deformation of the toner caused by the stress which the toner receives in the developing unit. The vicinities of the surfaces of the toner particles are relatively hard and the insides of the toner particles are soft. Among others, the following toner particles have been disclosed (WO 09/044,726 A). The toner particles each have large toughness against an external factor at the time of the pressurization of the toner as a result of an improvement in adhesiveness between the core portion and shell layer of the core-shell structure.